1. Field of the Invention
This invention relates to spider springs for seismometers and the like, and more particularly to a method and apparatus for mass producing such springs while they remain attached to their sheets from which they have been etched out.
2. Description of the Prior Art
Seismometers or geophones of the type to which this invention relates has a coil-mass assembly and a permanent magnet assembly that are adapted to move relative to each other in response to earth vibrations. One assembly is stationary and is in engagement with and moved by the earth. The second assembly is supported from springs and, due to inertia, lags behind the movement of the first assembly. The coil assembly produces an output electric signal which is proportional to the earth's vibrations.
The springs used in geophones of the type to which this invention relates are known in the art as "spider" springs. Geophones utilizing spider springs are described in the patent literature, see for example, U.S. Pat. Nos. 2,754,435, 3,157,852, 3,239,804, 3,258,742, 3,344,397, 3,451,040 and 3,718,900.
These spider springs are etched out from very thin flat sheets typically made of a beryllium copper alloy having a thickness of about 4 mils. Most digital grade geophones 8 Hz. and higher now utilize double-ended, cantilever spider springs consisting of an inner ring, an outer ring, and a plurality of double-ended arms interconnecting the rings. Each sheet of copper is typically rectangular and the circular springs are etched out in rows. After etching, the springs are still held together by tie tabs to facilitate severing the springs from the sheet. There are a plurality of rows and each row includes a plurality of springs. After the individual springs are severed from a very thin copper sheet which has not yet been heat treated, they are very fragile. Hand manipulations of their rings and especially their arms are likely to cause considerable damage to their shapes.
In each such spring there is a need to displace the inner ring relative to the outer ring in an axial direction. Creating such a displacement, known in the art as a "preform", requires a preform fixture. The present practice involves stacking in the fixture the individual springs severed from the sheet, one on top of the other, with their arms properly oriented relative to each other. The lesser the number of springs in the stack, the better their preforming will be. On the other hand, the greater the number of springs is in the stack, the faster the preforming operation becomes. Loading between 10 to 15 individual springs into the preform fixture, can provide springs with an acceptable preform.
The inner parts of the preforming fixture are so structured that when it is closed, all the inner rings of the springs become positioned relative to their outer rings by substantially the same displacement. The closed fixture together with its springs are then heated in an oven or in a hot salt bath for about 1 to 2 hours at a temperature of about 600.degree. F., and then the hot fixture is plunged into a quench bath to temper the springs. This heat treatment of the individual springs in the fixture hardens the springs, as is well known in the art of heat treating. When the fixture is opened and the individual springs are manually carefully removed therefrom, to avoid their arms from becoming entangled, each spring is permanently preformed so that its inner ring is axially displaced from its outer ring by a desired distance, when the spring is in a free condition.
The drawbacks of the above described preforming process are well known. It is relatively slow because it requires for the individual springs to be stacked, one on top of the other, in the preform fixture with the spring arms being properly oriented relative to each other. Such stacking of the fragile springs while they are in their non-heat treated state, requires considerable time and care. Any excessive twisting of the spring arms can produce unpredictable and undesirable output signals from the geophones utilizing them. This is so because the arms of the springs will flex unexpectedly. It may be anticipated that between 15% to 25% of the springs will become damaged during the present preforming process, either while loading the individual springs into the preform fixture, or while separating the individual springs from the fixture after the heat-treating-and-preforming process has been completed.
For that reason it is very important that the springs be individually graded and measured. If the springs are spot checked at random, defective springs will be discovered only after they have been assembled into the geophones. It can be expected that the distortions in the output signals from such defective geophones will fall outside of the prescribed tolerance limites. Manually grading the individual springs is another very time-consuming operation.
In certain types of geophones, the springs in addition to serving as a suspension system between the coil-mass assembly and the permanent magnet assembly, also serve as current conductors. In that case, it may be desired to plate each spring with a good conductive material, such as gold, silver or rhodium. This is accomplished by loading the preformed individual springs into a plating rack, which again is a time-consuming operation.
It is a general object of the present invention to greatly simplify the present preforming process for making spider springs. This is generally accomplished by eliminating most of the presently required hand manipulations especially while the springs are in their non-heat treated state. As a result, the percentage of deformed springs obtained with the invention is relatively small compared to the number of defective springs resulting from the presently utilized preforming process.
It is a further object of the invention to greatly increase the number of springs that may be preformed and heat-treated in a single fixture, thereby appreciably reducing the cost of fabricating seismometer spider springs from several sheets preformed simultaneously in a single fixture.
It is yet another object of the invention to improve the accuracy and consistency in the preforming process for spider springs.